Processes for the catalytic production of carbon from hydrocarbon gases and CO were proposed and patented as early as 1920 (U.S. Pats. No. 1,352,162; 1,868,921; 1,882,813). These patents identified the product as "carbon black," but it is clear from the experimental conditions that filamentary carbon was formed. The preferred catalysts were iron, cobalt and nickel or their oxides.
More recent work on filamentous carbon synthesis has been reported by a number of academic and industrial organizations throughout the world. Baker and Harris published a comprehensive review of the field in 1978 ("Chem. and Phys. of Carbon," 14, 83-165 [1978]). Most of the work has centered on the use of Fe as the hydrocarbon decomposition catalyst, although many of the patent disclosures claim Group VIII metals in general. The preferred gases are CO, the C.sub.1 -C.sub.3 alkanes and benzene, but much broader classes of hydrocarbons are often claimed.
Baker and co-workers have carried out extensive studies on the catalytic formation of filamentous carbon, by decomposition of acetylene at temperatures between approximately 500.degree. to 975.degree. C. in the presence of Fe, CO and Cr catalysts supported on single crystals of graphite and silicon ("J. Catal." 30(1), 86-95 [1973]), or over nickel films ("J. Catal." 26(1), 51-62 [1972]). Each of the filaments was observed to have a catalyst particle as its growing tip, where the diameter of the filaments was fixed by that of the catalyst particle. The filaments, diameter and length varied respectively between 0.01-0.15 microns and 0.5-8.0 microns. Filament growth followed random paths forming loops, spirals and other shapes. Growth rate varied inversely with catalyst particle size. The filaments stopped growing when the catalyst particle was completely covered with a carbon layer. Baker also studied the formation of carbon filaments from other hydrocarbon gases such as ethylene, benzene, 1,3-butadiene, allene and propyne ("Carbon," 13(3 ), 245-6 [1975]).
U.S. Pat. No. 4,565,683 (D. J. C. Yates and R. T. Baker) discloses FeO as a catalyst for carbon filament synthesis. The FeO, formed by steam treatment of Fe at 700.degree. C., is reacted with acetylene or ethane at 700.degree. C.
U.S. Pat. No. 3,816,609 discloses a process for the production of a hydrogen-rich stream from a hydrocarbon feed gas such as propane. The hydrocarbon feed is first converted to filamentary carbon using a supported Group VIII non-noble metal catalyst. The carbon is then gasified using steam to produce the hydrogen-rich gas stream.
U.S. Pats. No. 4,435,376 and 4,518,575 are directed to the synthesis of filamentary carbon from hydrocarbons and a (Ni,Ti)-based catalyst which has been promoted with phosphorus. The addition of phosphorus is claimed to result in filaments of decreased diameter and length and increased surface area, such that the "microfibrous carbon" is a good candidate as a reinforcing agent.
Department of Energy Report No. DOE/MC/14400-1551, described a process for making filamentary carbon by the catalytic reduction of a carbon-containing gas using iron as the catalyst. In one preferred embodiment of the process, carbon is deposited on an iron-based catalyst from a CO/hydrogen gas mixture in the 300.degree.-700.degree. C. temperature range at a pressure of 1-100 atmospheres. The carbon produced is called "ferrous carbon" and is described as fibrous, particulate material in which the metal catalyst particles are intimately dispersed as nodules throughout the fibrous carbon growth.
Koyama and Endo have developed a process for growing graphitic fibers at about 1000.degree. C. in which a gaseous mixture of benzene and hydrogen is passed through a reaction pipe coated with very fine particles of Fe (Japan Economic Journal, 17 [December 1981]). The fibers are reported to grow in a two-stage process (J. Crystal Growth 32(3), 335-349 [1976]). The growth process begins with the catalytic formation of very thin filaments which are then thickened by the pyrolytic deposition of carbon. The carbon fibers are typically 10 microns in diameter and several cm long. A 1982 Showa Denko K. K. patent (Japanese Kohai No. 57/117622) discloses that carbon filaments may be prepared by carbonizing a gaseous mixture containing benzene and hydrogen at 1000.degree. C. in the presence of Fe particles with a particle size less than 0.03 microns or the use of a suspension of Fe particles sprayed into a reaction chamber at 1000.degree. C. with a flowing mixture of benzene and hydrogen (Japanese Kohai No. 58/1180615).
G. G. Tibbetts and co-workers at General Motors developed processes for the growth of carbon filaments using methane or natural gas as the hydrocarbon gas at about 1000.degree. C. Catalyst particles are obtained from carburized stainless steel tubes (U.S. Pat. No. 4,391,787) or by wetting the inside of steel tubes with an aqueous ferric nitrate solution ("Carbon" 23(4), 423-430 [1985]), or by growing a thick layer of oxide on the inside of the tube (U.S. Pat. No. 4,497,788). Also disclosed is a process for growing graphite fibers on a ceramic substrate pretreated by evaporating a ferric nitrate solution to deposit an iron compound (U.S. Pat. No. 4,565,684). In the first of two carbon-growth stages, a mixture of 5-15 vol% methane and hydrogen is passed over the ceramic heated to between 600.degree.-1200.degree. C. (preferentially 1000.degree.-1100.degree. C.). During this stage catalytic growth of thin carbon filaments occurs. The second growth stage is then initiated by increasing the methane concentration in the gas to 25 vol% of higher. This results in the thickening of the filaments due to pyrolytic deposition of carbon into fibers with diameters between 5-15 microns and 1-3 cm long.
In 1974 Nishiyama and Tamai ("J. Catal.," 33(1), 98-107 [1974]) reported the formation of fibrous carbon on Ni/Cu alloy sheets and alloy powders from the decomposition of benzene in the 580.degree.-900.degree. C. temperature range. For both the sheet and powder cases, a large number of metallic particles were present in the carbon possessing the same composition as the substrate. For the catalysts in both the sheet and powder form, the deposition rate was higher for the alloys containing 40-80% Ni than for pure Ni. In some follow-up work in 1976 the authors reported on the beneficial effect of adding hydrogen to the benzene stream on the rate of formation of the fibrous carbon under certain conditions ("J. Catal." 45(1), 1-5[1976]).
In 1985, Bernardo et al ("J. Catal." 96(2), 517-534 [1985]) studied the deposition of carbon on silica supported Ni/Cu catalysts from a methane-steam mixture at 500.degree.-900.degree. C. The carbon deposits from alloys with 50-100% Ni were filaments with a less dense core and a metallic particle at the tip.
Neither Nishiyama and Tamai, nor Bernardo et al. discovered the surprising space-filling capability of filamentary carbon growth from Cu/Ni catalysts when ethane or ethylene are used as the hydrocarbon feed gas in the temperature range 500.degree.-700.degree. C. Neither did these workers report the primarily bi-directional, and at times multi-directional, growth pattern characterizing the process of this invention.